1. Field of the Invention
In general terms, the present invention relates to materials science, high-performance materials, ceramic composites, and carbon nanotubes. More specifically, the present invention is for a composition of matter and for a processing method comprising nanostructured carbon materials and/or vapor-grown carbon fibers (VGCFs) and a ceramic material wherein the resulting nanocomposite materials have enhanced structural and thermal barrier properties.
2. Description of the Related Art
Ceramic materials possess many attractive properties, such as good high temperature mechanical strength, high resistance to creep, resistance to oxidation and chemical attack, but they still need improved toughness values in order to make them advanced structural materials. Some researchers have already shown improved toughness values for silicon nitride. (Nakano et al., “Fabrication and Characterization of Three-Dimensional Carbon Fiber Reinforced Silicon Carbide and Silicon Nitride Composites,” J. Am. Ceram. Soc., 78(10), 2811-2814 (1995); Guo et al., “Carbon fibre-reinforced silicon nitride composite,” J. Mater. Sci. 17, 3611-3616 (1982); Grenet et al., “Carbon Fibre-Reinforced Silicon Nitride Composites by slurry Infilitration,” Ceramic Transactions, 58, 125-130 (1995)) through the use of carbon fibers as reinforcements. Presently, ever since the discovery of carbon nanotubes (CNTS) (Iijima, “Helical microtubules of graphitic carbon,” Nature, 354, 56-58 (1991)) their use as reinforcements in composite systems has been the focus of many research efforts (Thostenson et al., “Advances in the science and technology of carbon nanotubes and their composites: a review,” Composites Science and Technology, 61, 1899-1912 (2001), incorporated herein by reference). CNTs are the ideal fiber reinforcement for composite systems because of their outstanding mechanical properties (Yakobson et al., “Nanomechanics of carbon tubes: instabilities beyond linear response,” Physical Review Letters, 76(14), 2511-2514 (1996); Walters et al., “Elastic Strain of Freely Suspended Single Walled Carbon Nanotube ropes,” Applied Physics Letters, 74(25), 3803-3805 (1999)), flexibility (Falvo et al., “Bending and Buckling of Carbon Nanotubes Under large Strain,”Nature, 389, 582-584 (1997)), and electronic properties (Wildoer et al., “Electronic Structure of Atomically Resolved Carbon Nanotubes,” Nature, 391, 59-62 (1998)). They have a very high aspect ratio (length-to-diameter ratio) but are short enough to flow through conventional processing equipment, such as in the fabrication of polymer matrix composite systems (Calvert, “A Recipe for Strength,” Nature, 399, 210-211 (1999) and, as will be discussed later, CNT/ceramic composite systems fabricated by robocasting. CNTs can also serve as a new reinforcement to improve the brittleness of ceramic materials. Ma et al., “Processing and properties of carbon nanotubes-nano-SiC ceramic,” Journal of Materials Science, 33, 5243-5246 (1998), incorporated herein by reference, have fabricated-carbon nanotube-nano-SiC ceramic composites via a hot-press method. Three-point bending strength and fracture toughness values of these composites show a 10% increase over the monolithic SiC ceramic, which was fabricated under the same process. Further increases in toughness are possible if lower temperature processing is used (Zhan et al., “Single-wall carbon nanotubes as attractive toughening agents in alumina-based nanocomposites,” Nature Materials, 2, 38-42 (2003), incorporated herein by reference).
Piegney et al. developed a novel catalysis method for the in situ production of dispersed CNT bundles within a CNTs-Fe-Al2O3 composite powder (Peigney et al., “Carbon Nanotubes in novel Ceramic Matrix Composites,” Ceramics International, 26, 677-683 (2000), incorporated herein by reference). The composite powders were then hot-pressed at 1475° C. in a primary vacuum and, after hot-pressing, evidence of CNT bundle survival was found and the mechanical properties of the ceramic were retained (Flahaut et al., “Carbon Nanotube-Metal-Oxide Nanocomposites: Microstructure, Electrical Conductivity and Mechanical Properties,” Acta Mater., 48, 3803-3812 (2000), incorporated herein by reference). Hwang and Hwang (Hwang et al., “Carbon nanotube reinforced ceramics,” J. Mater. Chem., 11, 1722-1725 (2001), incorporated herein by reference) have studied the dispersion of CNTs using surfactants in order to achieve high levels of dispersion in their CNT reinforced ceramic system. Their study shows that surfactant molecules and CNTs form co-nicelle structures, which they used as templates to synthesize SiO2—CNT microrods that were used as reinforcements to inorganic ceramics. Their mechanical strength measurements of silicon dioxide ceramic were enhanced by ˜100% in the presence of ˜6 wt. % of CNTs.
Single-wall carbon nanotubes (SWNTs) possess highly anisotropic thermal properties. Theoretical calculations as well as some initial experimental efforts (Hone et al., “Electrical and thermal transport properties of magnetically aligned single wall carbon nanotube films,” Appl. Phys. Lett., 77, 666-668, (2000); Walters et al., “In-plane-aligned membranes of carbon nanotubes,” Chem. Phys. Lett., 338, 14-20 (2001)) have demonstrated that the longitudinal thermal conductivity is very high and within an order of magnitude of graphite or diamond (−2000 W/mK). In the transverse direction, the thermal conductivity is low and similar to that of C60 (0.4 W/mK) (Yu et al., “Thermal conductivity of single crystal C60,” Phys. Rev. Lett., 68, 2050-2053 (1992)). Rao et al. (Rao et al., “Zirconia nanotubes,” Chem. Comm., 1581 (1997), incorporated herein by reference) demonstrated that multi-walled carbon nanotubes could be coated with zirconia precursors and then burned out to leave behind nanoscale ceramic template structures.
Ceramic materials are also widely used as thermal barrier materials due to their refractory nature and low degree of thermal conductivity. Ceramics, however, are inherently brittle. Adding components to the ceramics which will enhance the materials' toughness while maintaining or enhancing the thermal barrier properties of the material would be a significant and useful advance. While such a decrease in the thermal conductivity of a thermal barrier material is alone worthwhile, the combination of decreased thermal conductivity coupled with increased toughness associated with the ceramic nanocomposites of the present invention represents a substantial improvement in the art of thermal barrier materials, particularly those that must perform in demanding applications such as those found in power turbine machinery. To date, there is no eligible prior art which discloses putting fullerene materials in ceramic matrices to form nanocomposites which have enhanced thermal barrier properties relative to the ceramic precursors. The present invention is directed towards such nanocomposites such that they can have a superior thermal, mechanical, and electrical properties as appropriate for particular applications.